The present invention relates to a twin roll strip casting device for casting the strip directly from a molten metal, and more particularly to an apparatus and a method for controlling a thickness of the strip in a twin roll strip casting device which can predict and compensate the thickness deviation of the strip caused by the eccentricity of roll and the movement of center of the roll, while maintaining the uniform gap between rolls in the casting process.
Generally, a twin roll strip casting device is used for directly casting a strip 5 by the rotation of the casting rolls 1 and 2 within a molten iron pool 3. In this case, the thickness of the cast strip 5 is dependent upon the gap between the rolls 1 and 2, i. e. the minimum distance between the rolls 1 and 2, roll nip.
To maintain the uniform thickness of the strip 5 in the twin roll strip casting device, therefore, the distance between the rolls 1 and 2 should be kept at a uniform distance.
To manufacture the desired thickness of strip, the thickness of the strip should be accurately measured, but a conventional measuring method using a contact sensor has the following disadvantages. During casting of the strip, since the temperature of the strip is very high, it is impossible to measure the thickness of the strip with this contact sensor. Since the failure of the thickness measurement of the strip means the failure of the measurement of the gap between the rolls, the gap between the rolls can not be measured accurately. Accordingly, a contact sensor 45 may be mounted between chocks 44 of rolls 41 and 42 to measure the gap between the rolls 41 and 42 so as to control the thickness of the strip, as shown in FIG. 4.
The gap between the rolls 41 and 42, that is, the thickness of the strip means the distance of the roll nip 46 as a minimum distance between the fixed roll 41 and the horizontal moving roll 42. In the conventional method, this means that only the gap between the chocks may be measured to measure the thickness of the strip instead of practical gap distance between the rolls. As a result, the conventional method is an indirectly measuring method.
In the conventional method for measuring the gap between the chocks 44, therefore, since the variation of the gap between the rolls 41 and 42 caused by the eccentricity of the rolls in the casting process and the upper/lower and left/right movements of the rolls 41 and 42 caused by the movements of the centers of the rolls can not be detected when rotating the rolls, the information related to the variation of the roll gap and the movement of the rolls cannot be utilized for measuring of the thickness of the strip. Therefore, the accuracy for measurement and strip thickness is deteriorated.
To overcome the above disadvantages and problems, a roll eccentricity compensation system has been introduced, in which the error value of the thickness of the strip is compensated using the roll separation force(RSF) of rolls caused by the eccentricity of the rolls during the rotation of rolls. However, since the RSF of the roll is created due to various kinds of factors such as the change of casting velocity, the change of the gap between the rolls, the change of the height of the molten pool, and skull flowing between the rolls, there occurs a problem that the RSF is not effective. Moreover, a method of compensating the variation of the thickness of the strip caused by the movements of the centers of rolls is not yet suggested in the conventional roll eccentricity compensation system.
An object of the present invention is to provide an apparatus and a method for controlling thickness of the strip in a twin roll strip casting device which can predict and compensate the thickness deviation of the strip caused by the eccentricity of rolls and the movements of centers of the rolls, while maintaining the uniform gap between the rolls in the casting process.
In order to achieve this object, the apparatus according to present invention comprises a fixed roll and a horizontally movable roll, a first sensor attached on a journal to measure an amount of variation between the journals of the fixed and horizontally moving movable rolls, second and third non-contacting sensors each mounted on the rear side of the barrels of the fixed and horizontally movable rolls to sense movements of the barrels of the fixed and horizontally movable rolls, first and second subtracters for each subtracting the amount of variation between the journals of the fixed and horizontally movable rolls which is sensed by the first sensor from the movements of the barrels of the fixed and horizontally movable rolls which are sensed by the second and third sensors, a controlling unit for processing input signals from the first and second subtracters to calculate an amount of variation of roll nip to eliminate a high frequency component from the calculated signal, and a roll gap controlling unit for controlling the gap between the rolls in accordance with the input signal of the controlling unit.
Preferably, the controlling unit comprises first and second buffers for each storing output signals from the first and second subtracters and for inverting the phase of the stored signals by 180xc2x0 to output the phase-inverted signals, first and second adders for adding the amount of variation between the journals of the rolls which is sensed by the first sensor to each of the output signals from the first and second buffers, a third subtracter for subtracting the output signal of the first adder from the output signal of the second adder to thereby calculate the amount of the variation of the roll nip, a gap trim predictor for generating an error compensating signal by the signal to be inputted from the third subtracter, and a fast Fourier transformer for performing Fourier transform for the error compensating signal from the gap trim predictor to output the transformed signal out of which the high frequency component is eliminated.
The roll gap controlling unit includes a fourth subtracter for adding the error compensating signal from the fast Fourier transformer to a desired value of the roll gap and for subtracting a measured value of the roll gap from this added value, a roll gap measuring sensor mounted between the chocks of the rolls to measure the roll gap between the chocks, a PID controller for outputting a control signal to increase the roll gap if the desired value of the roll gap added to the error compensating signal is higher than the measured value of the roll gap, and to decrease the roll gap if lower, in accordance with the compared result of the fourth subtracter, and a servo valve operated according to the control signal from the PID controller to move the movable roll.
Further, a control method for the thickness of the strip having a fixed roll and a horizontally movable roll includes the steps of measuring a movement value Gj(xcex8) of journals of the fixed and horizontally movable rolls and a movement value Gg(xcex8+xcfx80) of barrels of the rolls, predicting a movement value Mfcr(xcex8) of a roll nip of the fixed roll and a movement value Mmcr(xcex8) of a roll nip of the movable roll from the movement values Gj(xcex8) and Gg(xcex8+xcfx80); calculating a difference value between the movement values Mfcr(xcex8) and Mmcr(xcex8) to obtain an amount of gap variation Mdiff(xcex8) between the roll nip, and controlling thickness of a strip to minimize the amount of variation Mdiff(xcex8) of the gap between the roll nip.